Intergranular oxidation of Ni-base alloy 718 with a focus on additive manufacturing

•Additively manufactured alloys 718 show an excellent oxidation resistance in isothermal conditions.•TiNbO4 is identified by EBSD inside the Cr2O3 oxide scale, in agreement with thermodynamics.•AM samples present intergranular oxidation kinetics similar to the wrought alloy's but with deeper occurrences.•The larger the grain size, the thicker the oxide at the alloy grain boundaries.•Intergranular oxidation kinetics is controlled by both volume Al diffusion and interfacial oxygen diffusion. The intergranular oxidation in air at 850 °C of alloy 718 produced by laser beam melting and electron beam melting was compared to that of the wrought alloy. Quantitative analyses revealed that the amount of grain boundary oxidation was similar for these alloys. However, the additively manufactured ones presented deeper and thicker oxides at grain boundaries, due to grain size heterogeneity and to a smaller number of special boundaries. Results show that intergranular oxidation kinetics follows Wagner’s theory on internal oxidation considering not only O diffusion at the intergranular oxide/metal interfaces but also Al and Ti diffusion in the bulk.